1. Field of the Invention
The present invention relates to a brake-clutch assembly for use in a torque transmitting apparatus such as a winch.
2. Description of the Prior Art
There is in the prior art a winch manufactured by Warn Industries, Inc. of Kent, Washington, the assignee of the present invention, which comprises a motor that transmits power through a speed reducing gear transmission to a first drive cam having an axially facing cam surface. This drive cam engages an axially facing cam face of a second driven cam which through a further speed reducing gear transmission drives the cable winding drum of the winch. A clutch-brake assembly is operatively connected between the two cams, this assembly comprising a first shoe connected to the first cam to rotate therewith, a second shoe connected to the second cam to rotate therewith, and a ratchet plate positioned between the two shoes. There is a pawl which engages the ratchet plate to permit the ratchet plate to rotate only in the direction which the brake-clutch assembly moves to reel in cable.
When the winch is operated under power in a first direction to reel in cable, the drive cam engages the driven cam in wedging engagement to tend to move the two cams axially away from each other and to push the two shoes into engagement with the ratchet plate. This effectively locks up the two cams and the clutch-brake assembly as a single rotating unit through which power is transmitted to the drum to reel in cable.
When the winch is operated in the opposite direction, e.g. in lowering a load, the drive cam moves in a direction out of wedging engagement with the second cam, so that it does not push the shoes of the assembly into engagement with the ratchet plate. However, a circumferentially facing shoulder of the drive cam engages a matching shoulder of the driven cam so that the driven cam is positively engaged to be moved in a direction to unwind cable. If the cable is under tension loading, as in the instance of lowering a suspended mass, the tension load on the cable tends to cause the second driven cam to overrun the first drive cam to push the drive cam back into wedging engagement. Since the pawl engaging the ratchet plate does not permit rotation of the ratchet plate in a direction to unwind cable, as the two shoes come into frictional engagement with the ratchet plate, the ratchet plate tends to stop the rotation of the two shoes until the motor turns the drive cam sufficiently to cause it to "catch up". In effect, the brake-clutch assembly acts as a governor or "dynamic brake" to limit the rotational speed of the components to that which the motor imparts to them. In actual operation this condition stabilizes so that the drum rotates only at the speed permitted by the rotational speed of the motor, with the major torque loads exerted back through the winch components being absorbed in the sliding frictional engagement of the shoes with the ratchet plate.
In the prior art winch described above, the frictional engagement between the ratchet plate and the two shoes was provided by fixedly mounting to each face of the ratchet plate a number of arcuate sections of durable material (e.g. an asbestos based material), which is conventionally used for the facing material in brakes and clutches. However, this arrangement which is usually satisfactory in conventional brake or clutch assemblies did not prove entirely satisfactory in this particular application, where in the cable unwinding mode, the two shoes were in constant sliding frictional engagement with the ratchet plate, with the major portion of the energy input into the winch (for example, the energy resulting from lowering a load at a constant rate) being absorbed at the frictionally engaging surfaces of the two shoes and the ratchet plate. There was excessive wear, premature wearing out of the frictional engaging components, elevated temperatures, and worn firction material acting as a lubricant between the shoes and the ratchet plate. This was due to irregularities or misalignment of the frictionally engaging parts.
Within the broader field of brakes and clutches for various applications, there is in the prior art a variety of arrangements to provide frictional engagement between rotating parts. For example, in U.S. Pat. No. 1,734,598, Schramm, there is a driving member having a face which engages a plurality of cylindrical plugs which are loosely mounted in the peripheral portion of a plate. These plugs provide frictional engagement between the driving member and the plate so that power can be transmitted therebetween. A similar arrangement is shown in U.S. Pat. No. 2,082,696, Myers, where there are a number of cylindrical members, called "friction blocks" loosely positioned within a driven member.
U.S. Pat. No. 1,132,958 shows a plurality of projections on friction discs. In two other patents, U.S. Pat. No. 3,034,365, Stieber, and U.S. Pat. No. 3,224,540, Straub, there are a number of rollers freely mounted in disc plates to provide frictional engagement between members.
While the prior art does provide a number of brake mechanisms and clutch mechanisms which do provide frictional engagement between members in various arrangements, there is still a need for a clutch-brake assembly which can operate effectively in an apparatus such as the winch described above, particularly in its operating mode of a dynamic brake where it acts as a governor, and not experience excessive wear. Thus, it is an object of the present invention to provide a clutch-brake assembly to fulfil this need.